Wet areas in gardens

Updated on November 8, 2009

Handling a swamp

Wet areas in gardens can be expensive, and particularly
thankless places to do any form of landscaping. They can easily create problems
in the forms of wasted labor, failed plants and disruptions to local drainage.

Generally speaking the wet areas
are either places too close to the water table, areas subject to heavy runoff
which can’t drain, or near aquifers releasing ground water. Either way, they’re
a result of the local landforms and need to be approached cautiously.

Major remedial work on low lying
land or land subject to runoff can be extremely expensive, and in some cases,
ineffective. Usually the remedy is to build up the land, but by definition, the
new soil structure can be undercut by water if it can’t cope with the amount of
water or the nature of the flow. In some cases “remedial” landscaping can
create more problems, weakening soil structures or exposing new areas to water
flow. Areas containing clay are especially difficult. Clay is pre-bonded to
water molecules, and will pass on excess water very destructively, in addition
to making a viscous and uncooperative soup of itself.

The general rule with any wet
locale is to assess and plan. Band aid solutions will not work. Time and money
can be soaked up pretty easily, even if the water isn’t.

Plants are a good alternative to
big structural work. There are many plants which can thrive in wet areas, and
act as drainage. Big plants can take up a lot of water. They also act as a base
for soil structure, preventing the water from breaking it up, and giving
dislodged soil somewhere to build. Plants
effectively manipulate soil structures over time.

In choosing plants, the locality is
the primary consideration. Wetland flora varies immensely. In some places
native reeds are the main inhabitants of wet areas, in others trees are the
primary vegetation. One of the reasons for this is that wet areas contain soil
biota which can destroy other plants. Those that have adapted to the wet
conditions often have a monopoly of the environment.

In all wet areas, fungi are
rampant. They love moisture, and are efficient destroyers of plants that can’t
handle them. When picking plants for a wet area, it’s therefore extremely
important to know what will grow in your area. The existing plants will be a
rough guide to what can survive in your area. Plants from other wet areas may
well thrive, but there’s always a risk that they’re specialists in their own
regions. What’s a wet area plant in one region might wind up as compost in
another. Most failed plantings are as much a result of losing the argument with
the soil biota as any other reason.

Usually, trees are a yes/no
option. Forget the dry land trees. They just won’t survive. Some trees,
particularly river flora, can handle just about any wet conditions, and do
well. Even so, putting in a hundred trees can be over- betting to a terminal
degree. Try an experimental seedling or so from different species. Things like
willows are usually excellent in wet areas, with a great capacity to absorb
water, but again, locality matters. Willows are almost un-killable, but any
gardener can tell you that “almost” isn’t always good enough. There’s a fairly
wide choice of wet area trees, some particularly beautiful. You can base your
selection on any form of aesthetics, but the result will be based on survival. See
if your seedlings can handle all four seasons. If so, and they’re growing well,
you can plant more.

Some plants are particularly
reliable in just about any wet area. Cycads, one of the oldest forms of life on
Earth, will thrive in nearly any wet area that isn’t salty. They’re big,
beautiful, and provided they have water, nearly indestructible. Some can really
be considered canopy, when they reach full size. They require no maintenance,
and are among the few plants where fungi aren’t a problem. They need their
space, but they do all the work.

Ferns are also good in wet areas,
but a bit neurotic, compared to cycads. Things like maidenhair will grown
magnificently in the right place, and cause pseudo-pattern baldness in the
wrong place, as the gardener tears out hair wondering what’s wrong. Again,
experiment with trial plantings. It saves time and effort.

Flowering plants in wet areas can
be a real nuisance. Experience will show, in ruthless detail, what happens to
the ones that don’t make it. The only flowering plants I know for certain can
handle just about any level of wetness are agapanthus and golden poker, and
some of their relatives. They’re very big plants, they clump, and can absorb
large amounts of water, as well as being pretty trustworthy in the dry periods.
They’re quasi-succulents, in some ways, with a good ability to retain moisture
in those times.

Ironically, it’s the occasional dry
periods that are the real threat to a wet area garden. Ferns loathe the dry spells. Cycads do a bit
better, but most wetland plants hate dry times. Soil structure changes, root
systems suffer from the setting soil, now much harder and more energy-expensive
for the plants to gather nutrients. Wetland plants have adapted to take
advantage of the mineral soup that those areas provide.

You can figure out what happens
in your area in dry times. Outside the wet area, what’s growing? Pines? Other
conifers? Things like oaks, maples, etc. real dry land plants? If so, they’re
the things that would be growing in your wet area, if it wasn’t wet. Usually
there’s a pretty distinct line of separation between wet and dry growth. The
dry land trees are the best indication.

In a drought, or if the levels of
moisture in the soil change, they’re also stronger, and some of them are
colonizers. They’re good indicators of changes in the wet area. If dry area
plants appear in the wet, you’ve got the beginnings of a significant problem.
The area will need extra water. Sometimes gardeners can alter a wet area
unintentionally. It may be that some of
the wetland trees have altered the overall levels of moisture, (willows, in
numbers, are perfectly capable of doing that) and that the soil will now allow
dry land trees to grow.

This is the mangrove effect. Plants
alter the soil to the point of creating opportunities for other plants. It
takes years, but if you’ve ever noticed the odd mix of plants around some
areas, wet and dry area trees together, there’s your answer. In some cases they
do well together, but in others, where things like pines move in, the area is
undergoing a major shift in its flora. The pines will drive out other trees over
time. Soil biota also changes, naturally, and some of the organisms supporting
the wet area plants can’t survive in the reduced moisture levels. Nutrition available
to wet area plants also suffers from the relative dryness. Breakdown of
materials by fungi is also affected, so the entire chemical tenor of the area
is changed.

This can happen to wet area
gardens, and this is where the real gardening comes in. Wet area plants can
generally fend for themselves in their correct environment, but this sort of
thing is beyond them. To properly maintain a wet area garden you need to watch
growth habits and be sensitive to any sign of drying out. Generally you can
handle reduced moisture with something pretty simple, like a good drip
irrigator, very economical, and easy to maintain.

Weeding is another mixed blessing
of the wet area garden. Rule: if you don’t know what it is, or what it’s doing
there, get rid of it. Wet area plants can include some destroyers, very
invasive water plants which can suffocate your plantings. Be ruthless. The
plants I’ve suggested are pretty reliable, but some weeds can overrun an area
very quickly, and really mess up a good wet area garden. Aesthetically, they’re
a nuisance, and in terms of new growth from your plants, they’re a liability.

Fertilizer: In wet areas, not
much is required. They tend to be areas where nutrients are provided by water
movement, like river silt. Experienced gardeners will know that they can also
encourage weeds like nothing else. If you have specialist plants, like wet
region orchids, non-epiphytes, in particular, some considered fertilization is
probably needed, but for everything else, less is definitely better. You can
always add a bit. Don’t get too fussy about NPK mixes, or other technicalities,
because they are subject to the water movement as well. A good equally balanced
mix will be enough, and makes sure you don’t overdo anything. Too much
phosphate isn’t good, if the other two can’t back it up. Too much nitrogen, and
you get a lot of foliage, and maybe not much else. Let the plants figure it
out. If flowerings are low, OK, maybe some of a specific element, for that
plant. If growth is iffy, ditto. Common sense, and avoiding unnecessary labor,
will achieve more with less effort.

Pests and diseases: Wet area
plants have one added difficulty; water is an ideal common medium for the spread
of pathogens and pests. Water is also inclined to make good mixing solutions
for pesticides and herbicides.

Insect pests should be dealt with
locally, using some care not to saturate the area with pesticides. The area
should be allowed to flush clean of any residues. This means avoiding, if at
all possible, high concentrations of pesticides as a regular procedure. They can
and do kill benign organisms.

Diseases should be dealt with
using the one iron law of gardening; remove infected plants, immediately, and
destroy them. Particularly if the plant has become suddenly severely affected. Decontaminate
the area, if necessary to the extent of replacing soil. Keep a close eye on any
plants around the area of infection. Wet areas are vulnerable to rapid spread
of waterborne organisms, and the entire area can suffer terribly. That said,
some sprays can be as bad or worse. Some diseases are spread by insects as
vectors, and this is where the conjunction of insecticides and other sprays can
be lethal, and not only to the plants. Hormone based insect prays are extremely
controversial, and with good reason. They can damage an ecology severely, seriously
affecting other organisms, and linger for decades. Whatever you use, make sure
it’s manageable, and as per fertilizer, less is better.

DO NOT make weird and wonderful
cocktails of sprays, either deliberately or as a result of concurrent
treatments of garden problems. Neurotoxins, fungicides, hormone sprays and
other exotica are not a synonym for “safe for baby”. If you were looking for a
supply of dangerous chemicals, with unknown possibilities when used in
combination, these are the sort of chemicals which would really get a coroner
interested. Wear gloves, space suits, whatever, but do not underestimate the
ability of these very poisonous compounds to do you damage.

Wet areas have the advantage of
being able to flush themselves clean of toxins, but it can take time. One possibility
is to just spray for insects, and manually remove any diseased or weakened
plants, rather than simultaneously spraying for a disease. The difference for
the gardener is that the water in the soil will spread the chemicals very rapidly.
It does dilute them, but they’re pretty powerful chemicals, and work on very
low, aerosol-type, in some cases, molecular, dosages.

General maintenance: The best
defence for any garden is healthy plants. Poor growth indicates either an
unhealthy plant, or an unhealthy environment. Your garden will show you what’s
happening, in detail. Keep your eyes open and be alert for anything which looks
wrong, or not as it should be. Make sure you know exactly what you’re looking
at, before you do anything, if you’re not sure. Advice is useful, but certainty
is better.

Wet area gardens can be truly
beautiful. They can be spectacular. Enjoy your garden, and learn from it. I
promise you, it doesn’t get dull. Ever.

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Comments 3 comments

We have a wet area in one of our flower beds, and I'm still experimenting with plants there. You're so right--it's the dry times that are the challenge. Thanks for the ideas!

Marcos 23 months ago

Hi I have successfully grown toatmo plant from seed they are kept in my greenhouse as they are in large pots I do make sure they are watered well as they tend to dry out quickly and the toatmos are doing well. also feed every 4 days but I will be away on holiday would it be ok to bring the the plants outside the greenhouse thank you for advice and also for my toatmo club news emails